What might cause swollen lymph nodes in an animal?

Swollen lymph nodes, also known as lymphadenopathy, indicate an immune response to various stimuli within the body. The correct answer focuses on infection or systemic disease as primary causes. When an animal experiences an infection, whether it's localized or systemic, the lymphatic system works to filter out pathogens, leading to an increase in lymphocyte production and resulting in swollen lymph nodes. Common infections that can cause this include viral, bacterial, or even parasitic infections.

Systemic diseases, such as cancers (like lymphoma) or autoimmune disorders, can also trigger lymph node enlargement. This bodily response signifies that the immune system is active and attempting to fight off invaders or cope with disease processes, which is why this option is the most accurate in explaining the cause of swollen lymph nodes in an animal. This understanding is crucial for veterinary assessments, as swollen lymph nodes can be significant indicators of underlying health issues that require investigation and potential treatment.